Way too many vertices when switching to edit mode
Shrinkwrap modifier could help you:


Create new object with similar shape, big enough to cover your sculpted rock
Subdivide it as you would like the mesh of your rock to be detailed (or use Subdivision Surface modifier so you can tune it later)
Apply Shrinkwrap modifier with Target set to your high-poly sculpted rock.




You should get low-polygon version of your high-poly sculpted model.
There are some useful options you can use for tuning the Shrinkwrap modifier so you can look at it's overview in Blender Manual.

https://docs.blender.org/manual/en/dev/modeling/modifiers/deform/shrinkwrap.html